This invention relates to raise drills and, in particular, to the hydraulic system for operating such an apparatus. Raise drilling is a term which relates to a technique of boring or remaining large diameter holes which includes drilling a relatively small diameter pilot hole into earth strata until the cutting bit emerges into an open space and then replacing the small cutting bit with a specially-designed large-diameter reamer and cuttng the larger hole along the path of the pilot hole by pulling the reamer back toward the drill rig. This technique is well known in the art and many drill rig apparatuses have been developed.
The subject invention relates to two other applications filed on the same day herewith, Ser. Nos. 38,753 and 38,754, entitled "Raise Drill Apparatus" and "Chuck and Wrench Assembly for Raise Drill Apparatus", respectively. These applications are incorporated herein by reference for additional background information.
Most such drill rigs utilize hydrulic thrust cylinders for raising and lowering a drill head which itself is rotated by means of an electric or hydraulic motor. For the instant invention a hydraulic motor is used. A closed-loop type hydrulic circuit is normally used to operate hydrostatic-drive raise drills. The closed-loop circuit is one where the drive pump output is directed through valves to a motor and the motor return oil is directed back to the pump inlet. Additional cooled and filtered oil is added by a make-up pump at the pump inlet, resulting n contaminants cycling several times through expensive pumps and motors before being caught by filters. It is also difficult to divert flow out of the closed loop from the large hydrostatic drive pumps so that rapid traversing of the large volume thrust cylinders during rod changing can take place because the pumps tend to lose inlet pressure and cavitate.